The present disclosure relates generally to managing water heater systems. More particularly, it relates to managing and controlling water heater systems in a manner responsive to varying energy demand periods.
Water heater storage tanks are used for storing and supplying hot water to households. A typical residential water heater holds about fifty gallons (190 liters) of water inside a steel reservoir tank. A thermostat is used to control the temperature of the water inside the tank. Many water heaters permit a consumer to set the thermostat to a temperature between 90 and 150 degrees Fahrenheit (F) (32 to 65 degrees Celsius (C)). To prevent scalding and to save energy, most consumers set thermostat to heat the reservoir water to a temperature in a range between 120.0 degrees F. to 140.0 degrees F. (about forty-nine degrees C. to sixty degrees C.).
A water heater typically delivers hot water according to the thermostat temperature setting. As a consumer draws water from the water heater, the water temperature in the water heater usually drops. Any time the thermostat senses that the temperature of the water inside the tank drops too far below thermostat's set point, power is sent to the electric resistance heating element (or a burner in a gas water heater). The electric elements then draw energy to heat the water inside the tank to a preset temperature level.
In some locations of the United States and globally, the cost for electrical energy can vary as a function of the time of day, day of the week and season of the year. In areas of the United States where energy is at a premium, utility companies often divide their time of use rates into off-peak and on-peak energy demand periods with a significant rate difference between the periods. For example, energy used during off-peak hours may cost the consumer in United States dollars around 5 cents to 6 cents per kilowatt hour (kWh), while on-peak period energy may cost anywhere from 20 cents per kWh to $1.20 or more per kWh.
A water heater that heats based on the water demand of a typical household is likely to heat at the same time as when energy demand on a utility company is at its highest. As a result, drawing energy to heat a water heater during these on-peak energy periods increases a consumer's monthly energy bill. The disclosure seeks to provide a means to avoid on peak energy use, saving the consumer operating expense, while supplying a continuous supply of domestic hot water utilizing conventional and possibly existing electric water heating systems.
One approach to negotiate the utility companies' time of use energy rates would be to use a programmable timer to turn off the entire water heater or the lower element. For example, a clock timer could be used to provide planned heating periods during known off peak periods of the day. While this approach is possible, adapting to period variation in the rate schedule and emergency load shedding request signals from the utility are not accommodated.
Simply increasing the storage size of the tank and/or increasing the set temperature of the tank in combination with use of a thermostatic mixing valve at the hot water outlet, serves to increase the hot water capacity, but it does not alter the energy consumption pattern of the water heating system. The lower heating element will also need to be disengaged in order to avoid consumption during “on peak” energy rate hot water usage.
Set point alteration is another means to reduce heating events during on peak water usage. While this will produce a similar outcome as disengagement of the heating elements, it requires a substantially different control mechanism for regulation and limiting of the tank temperature and cannot be easily retrofit to an existing water heating system.
Another approach is simply shutting the entire water heater off during on peak energy periods. This could result in the consumer running out of hot water during peak hours and left to wait until off peak hours to resume heating the entire stored water volume of the tank, meeting demand. This approach requires consumer behavior change or purchase and installation of a larger storage tank size to bridge the peak hour water usage. This results in an investment requirement from the consumer and presumes the availability of space to install a larger tank. Commonly, space limitation prevents installation of a water heater large enough to meet the storage meets to bridge the peak hours.
A non-replenishing tank could be used to maintain heated temperatures during “on peak” hours and be refilled and heated only during off peak hours. However, this approach requires an open tank or a means to compensate for pressure and volume changes.
Copending U.S. application Ser. No. 12/623,753 describes a system which provides a continuous supply of domestic hot water to meet the needs of a consumer, while utilizing off peak hours for heating of the stored water. Such a system also provides a valuable mechanism for a utility to shed load during peak and critical power demand periods. Another aspect of said application is that the upper and lower heating elements can be enabled/disabled independently based on the demand response signal level. Still another aspect of the disclosure is the heating operation corresponding to the demand response level is consumer selectable for multiple tier signals (which may be greater than four levels). During low energy rate conditions, the lower element is engaged to heat the contents of the full tank for future use during high energy rate periods. The lower element is then disengaged during high energy rate periods according to the programmed schedule, or an external or consumer input, reducing energy consumption during high energy rate periods. A limitation of this system is that the stored energy can only be used for hot water. If the consumer is away, or not using water that stored, energy is essentially wasted.
Thus there is a need for a system that can remove excess energy from the hot water heater when energy rates are high and store additional energy when electric rates are low.